The Mountain West would accept them with open arms......and whoever else died on the vine in the PAC 12. That move would be best for the 4 schools left. It would strengthen that conference and raise their level of visibility and respectability. The addition of those current PAC schools would certainly raise the level of attractiveness to potential media partners.
The only other thing the PAC can do at this point is try and recruit other western region schools to join the "new" PAC 12, including those from the Mountain West. Good luck with that. Possible targets could be ......Pacific (no football team), San Diego St, USF (San Francisco), Portland, BYU, Sacramento St, Gonzaga, UNLV, Wyoming, Hawaii, Nevada, New Mexico, Fresno St. San Jose St, UC Davis, Weber St, etc. All mid-major schools with only "regional" fan appeal. The PAC is going to have to do something. They can't go forward with only 4 schools.